One type of header that is fitted to combine harvesters for harvesting grain comprises a reel that extends transversely to the direction of travel. An elongate cutterbar located beneath the reel cuts the stalks of the crop, spring tines projecting from the rotating reel engage the cut stalks and push them on to an auger that conveys the crop towards the centre of the harvester. From there, the crop is carried into the processing machinery of the harvester.
The optimal distance between the cutterbar and the auger depends on the type of crop being harvested. Therefore the cutter is mounted in such a manner that it can be moved forwards and backwards relative to the auger.
The cutterbar comprises a set of blades, movable in a reciprocated movement relative to stationary blade guides by means of a so-called wobble box. The cutter and its drive mechanism are mounted on a header sub-frame that can be moved relative to the main frame of the header by more than 50 cms, typically. The wobble box has a drive pulley that is driven by a belt which passes over a stationary powered pulley mounted to the main frame of the header. A belt configuration is therefore required which does not obstruct the movement of the cutter sub-frame, yet which maintains the drive belt under the correct tension to drive the wobble box in all positions of the cutter sub-frame.
The geometry of the belt drive for a movable cutter of a harvester is further complicated by the magnitude of the permissible movement. It is necessary to ensure that the different runs of the belt never touch one another in any position of the cutterbar sub-frame. Furthermore, the belt and its various drive, guide and tensioning pulleys must all remain protected within a housing that covers the entire drive mechanism to prevent entanglement of the crop.
Prior art attempts to meet the above criteria have resulted in belt geometries having as many as eight different pulleys and, despite their complexity, additional pivot points were needed in the frame to allow pulleys to be repositioned to achieve the desired belt tension in different positions of the cutter.
Nowadays extendable headers use one or two idlers/tensioners mounted on a pivoting arm. Said arm is mounted onto a shaft welded to the header frame. A disadvantage is that the belt can not be kept tight over the full range of knife displacement. For full forward movement of the header, the belt needs to be demounted and remounted along a different path. Furthermore, in order to keep the belt tight over the full extension range, a very long spring is needed.
EP 1,653,122 describes a belt drive for connecting a stationary pulley to a relatively movable pulley, comprising a first guide pulley mounted in a fixed position in relation to the stationary pulley and a second guide pulley mounted for movement with the movable pulley, wherein the second guide pulley is capable of a limited degree of movement relative to the movable pulley and is spring biased in a direction to maintain the belt in tension. The second guide pulley is mounted on a long arm welded to the wobblebox support and movable with the header. Such a system can keep the belt under tension, but it suffers highly from vibration problems. The long arm amplifies the vibrations of the header frame it is welded to. Such a system also creates problems for accessing the auger drive area.